High frequency antenna



G. H. BROWN NIGH FREQUENCY ANTENNA March 3, 1942.

Filed Nov. 24, 1939 0 RSSK whit l fe-d 6Q Snvcntor George H Brown ttorneg Patented Mar. 3, 1942 HIGH FREQUENCY ANTENNA George H. Brown,Haddonfield, N. J., assignor to Radio Corporation of America, acorporation of Delaware Application November 24, 1939, Serial No.305,813

14 Claims.

This invention relates to high frequency antennas, and more particularlyto an improved demountable antenna, the impedance of which is readilyadjusted to match the impedance of a conventional transmission line.

This invention is an improvement over the high frequency demountableantenna described and claimed in U. S. Patent No. 2,234,333, issuedMarch 11, 1941, to G. H. Brown et al., for Demountable antennas.

The antenna described in the above-identified patent utilizes a quarterwave concentric line to support a quarter wave antenna. A matchingtransformer is used to couple a standard 70-ohm transmission line to theantenna which has a resistive impedance of approximately 21.5 ohms. Ihave found that an antenna of this type may be made more compact,lighter, and considerably more economical by eliminating the couplingtransformer which has heretofore been deemed necessary. Themodifications to the concentric line and the antenna, which arenecessary as a result of the elimination of the coupling transformer,also produce a further reduction of size, as will appear subsequently.

Inasmuch as the 70-ohm transmission line has been standardized by theindustry and is commercially available, and since many transmitteroutput circuits are designed to accommodate '70-ohm lines, it isbelieved to be inadvisable to eliminate the coupling transformer of theantenna described in said patent by changing the impedance of the lineto match that of a quarter wave antenna. Nor can the antenna impedancebe changed to match the line impedance merely by changing its length,for it would then become reactive and its impedance would vary sharplywith frequency, thus producing reflections in the line at sidebandfrequencies.

It is the primary purpose of this invention, therefore, to provide meansfor adjusting the impedance of an antenna to equal that of a standardtransmission line, and particularly to adjust the impedance of anantenna of the type described in the above-identified patent for thesame purpose. Further objects of this invention are the provision of animproved demountable antenna, and the provision of a high frequencyantenna adapted for use at a given frequency within a wide range offrequencies.

This invention will be better understood from the following descriptionwhen considered in connection with the accompanying drawing; its scopeis indicated by the appended claims. Referring to the drawing, Figure 1is a perspective view of an embodiment of this invention; Figure 2 is aview, partly in section, of an embodiment of this invention; Figure 3 isa schematic diagram of an equivalent circuit; Figures 4-and 5 are curvesillustrating the theory ofoperation of this invention; and Figure 6 is aview of an embodiment utilizing a reflector and a director to obtain adirectional radiation pattern. Similar reference numerals refer tosimilar parts throughoutthe several drawings.

Referring to Fig. 1, a hollow metallic mast I supports the radiatingelement 9 and a nonradiating element If]. The radiating element is thatportion 9 of the antenna extending above.

the top of the mast I. The radiating element 9 is connected to and issupported by the nonradiating element which includes the upper portionof the mast 1 and the conductor I! which extends centrally into the mastand terminates in a shorting block [3. The shorting block i3 makes anelectrical connection between the mast I and the conductor H. Theantenna is fed by a concentric transmission line I5, having its outerconductor grounded to the mast 1 and its inner conductor connected tothe bottom of the radiating element 9, that is, to the junction of theradiating and nonradiating elements. As in the antenna described in saidpatent, four horizontal conductors ll extending radially from the top ofthe mast 1 may be used to provide an effective ground for the antenna.The arrangement illustrated in Fig. l is intended merely to illustratethe essentials of this invention and not to embody the requirements ofan actual construction. Its adjustment and operation will now beexplained.

The length of the antenna 9 is made slightly less than a quarterwavelength so that it has a capacitive reactance. The position of theshorting block 13 is then adjusted until the antenna is tuned toparallel resonance at the operating frequency. The shunt impedance mayalso be selected by a suitable choice of antenna length, as will now beexplained.

The impedance of a quarter wave antenna which is mounted above fourradial ground rods, as illustrated in Fig. 1, has a purely resistiveimpedance of approximately 21 ohms. Above and below a quarterwavelength, the antenna impedance becomes reactive, as shown in Fig. 4,to Which reference is nOW made. Fig. 4 represents the reactance vs.length characteristic of a radiating conductor, for a given operatingfrequency. It will be noted that the curve crosses the zero axis at eachmultiple of a quarter wavelength. Just below a quarter wave, forexample, the antenna reactance is negative, that is, capacitive, while,just above a quarter wave, the reactance is positive, that is,inductive. The curve is a cotangent curve modified by the finite valueof antenna resistance which prevents the antenna impedance from reachinga very high value on either side of resonance at the half wave position.The curve R represents the effective antenna radiation resistance.

The reactance vs. length characteristic of a concentric lineshort-circuited at one end is illustrated in Fig. 5, to which referenceis now made. This curve is approximately a tangent curve but, as before,the theoretical infinite reactance adjacent the quarter Wave position isnot reached due to its finite resistance. Since the resistance of aclosed quarter wave line is much less than the radiation resistance ofan antenna, however, the reactance becomes very much greater at theresonant point. In this case, it will be noted that the reactance ispositive, that is, inductive just below the quarter wave point.

Now the impedance looking from the transmission line into the antenna isequivalent to that.

of the circuit illustrated in Fig. 3 in which R is the antenna radiationresistance, C is the capacitive reactance of the antenna, assuming it tobe less than a quarter wave long, and L is the parallelinductivereactance of the supporting concentric line, assuming it also to be lessthan a quarter wave long.

The shunt admittance of such a circuit may be obtained by adding theadmittances of .the

where Y1 is the admittance of the antenna branch including R. and C, andY2 is the admittance of the concentric line, neglecting its resistance.

If the two reactive components are made equal, which is done inaccordance with this .invention by adjusting the relative lengths of theantenna and the concentric line, the input impedance Zin is seen to bethe reciprocal of the first term of Equation 1. Thus From Equation 3 ismay be seen that the capaci tive reactance of the antenna affects itsinput impedance at resonance. By making the length of the antenna alittle less than a quarter wave, or odd multiple thereof, it will have acapacitive reactance, and the actual value of capacitance may also becontrolled. Assume, for example, that the length of the antenna is madesuch that its reactance is represented by the point A in Fig. 4. Inorder to tune the shunt circuit to parallel resonance, the concentricline must be made inductive, and it is therefore shortened until itsreactance is represented by point B in Fig. 5. The input impedance atthe base of the antenna will be a pure resistance and. will have.

some value which is determined by the R and C of the antenna, asindicated in Equation 3. If it is found that this value of impedance istoo low for the transmission line impedance, increasing the length ofthe antenna will decrease C.

and increase R which will increase the input impeclance. The concentricline inductance is then corrected to return the system to resonance.

It will be found in practice that the measured length of the antenna andconcentric line will vary somewhat from the calculated value. This isdue to the capacitive efiect of insulators and clamps which arenecessary for structural reasons. For a given design, however, thefractional wavelength for the antenna and the concentric linecorresponding to a given input impedance, remain constant over a widerange of operating frequencies. For example, to match an antenna of thistype to a standard IO-ohm line, I have found that the antenna should be0.243 x and the concentric line resonator should be 0.119 A. Thedifference in their lengths is due to the different effect of theinsulator on the concentric line and the antenna, and the difference inthe behavior of the characteristic curve near the quarter wave point.That is, Fig. 4 shows that immediately below the quarter wave point, theantenna reactance is small, but immediately below the quarter wavepoint, the concentric line inductance is very large. Consequently, theline must be shortened more than the antenna to provide an equalreactance. This, however, is an advantage, since it reduces the weightand size of the concentric section.

Fig. 2 illustrates a practical example of an antenna which may beadjusted for operation in any desired frequency band. For any giveninstallation in which the operating frequency is fixed, however, it isrecommended that the construction be simplified by building theconcentric line resonator and the antenna to the required length. Thismay be determined for any frequency from the fractional wavelengthfigures given above. However, the adjustable arrangement shown isfrequently desirable.

The antenna is clamped to a metal or wooden mast I by a pair of clamps2| which are securely fastened to the outer conductor 23 of theconcentric line resonator ID. The inner conductor l l is also hollow,and includes means for slidably adjusting the length of the antennaradiator 9, which is clamped in position by a clamp screw arrangement25. An insulator l9 is located at the top of the concentric line to holdthe inner conductor in position. The shorting block I 3 may be adjustedby inserting a suitable tool in the bottom of the line. The transmissionline is connected to the base of the antenna by means of a clamp 21.

In case the concentric resonator I0 is built to the required length, itmay conveniently be mounted within the supporting mast, since noadjustment need be made.

Referring now to Fig. -6, we have shown an arrangement formaking adirectional antenna. Like the arrangement shown in Fig. l, theconcentric resonator is within the upper section of the metal mast 1,and the grounding rods H are also provided. In addition, a verticalreflector 29 is suitably mounted on a bracket 33. It is spacedapproximately 8 from the antenna. A vertical director 3| may also beemployed. The director is on the opposite side of the antenna from thereflector, and similarly spaced from the antenna. The length of thereflector is greater than that of the antenna, while the director ispreferably shorter than the antenna.

I have thus described a compact antenna which may be built to match theimpedance of a given transmission line, which is at ground potential tolightning and static discharges, and which may be conveniently mountedin or on a metallic or wooden supporting mast.

I claim as my invention:-

1. A high frequency antenna comprising a radiating element having oneend electrically connected to the inner member of a reactive concentricsupporting element, means for connecting the junction of said antennaand said supporting element to a transmission line of predeterminedcharacteristic impedance, the length of said radiating element and thereactance of said sup porting element being so selected that theimpedance of said antenna and supporting element at said point ofconnection is equal to said predetermined characteristic impedance.

2. A high frequency antenna comprising a radiating element having acapacitive reactance connected to a concentric supporting element havingan inductive reactance, means for connecting the junction point of saidelements to a transmission line having a predetermined characteristicimpedance, the reactances of said elements being selected tomatch theimpedance of said antenna at said junction point to the impedance ofsaid line.

3. A high frequency antenna comprising a radiating element and a closedconcentric line nonradiating element, means connecting one end of saidradiating element to the open end of the inner conductor of saidnonradiating element, means for connecting a transmission line to thejunction point of said elements, the reactance and resistance of saidelements being selected to match the impedance of said antenna totheimpedance of said line, said non-radiating element having an electricallength less than a quarter of the operating wavelength, or any oddmultiple thereof.

4. A device of the character described in claim 3 and in which saidradiating element is an extension of the inner conductor of saidconcentric line having an electrical length less than a quarter of theoperating wavelength, or an odd multiple thereof.

5. A high frequency antenna comprising a radiating element and anon-radiating element, said non-radiating element comprising aconcentric line section having means for adjusting its electricallength, and said radiating element comprising an extension of the innerconductor of said concentric line, means for adjusting the electricallength of said radiating element, and means for connecting atransmission line to the junction of said elements.

6. A high frequency antenna comprising a radiating element joined to anon-radiating element, said non-radiating element serving to support andto tune said radiating element to resonance at the operating frequency,and means for connecting a transmission line to the point of junction ofsaid elements.

'7. A high frequency antenna comprising a concentric line section havinginner and outer conductors, means slidably mounted on said innerconductor and engaging said outer conductor for adjusting the electricallength of said line, a radiating element connected to said innerconductor, means for adjusting the electrical length of said radiatingelement, and means for connecting a transmission line to the junction ofsaid radiating element and said inner conductor.

8. In a high frequency antenna having a reactive radiating element and areactive concentric line section connected to said radiating element forsupporting and resonating said element, said elements, being connectedat their junction to a transmission line, the method of adjusting theshunt impedance of said antenna to equal that of said line, whichcomprises adjusting the length of said radiating element and saidconcentric line section to resonate said element at the operatingfrequency, the reactances and resistance of said elements being selectedto produce an antenna impedance equal to the impedance of saidtransmission line.

9. A high frequency antenna comprising a radiating element connected toa reactive concentric supporting element, means for connecting thejunction of said antenna and said supporting element to a transmissionline of predetermined characteristic impedance, the length of saidradiating element and the reactance of said supporting element being soselected that the impedance of said antenna and supporting element atsaid point of connection is equalto said predetermined characteristicimpedance, and a plurality of quarter wave conductors connected to theend of the outer conductor of said supporting element adjacent saidradiating element.

10. A high frequency antenna comprising a radiating element having acapacitive reactance connected to a concentric supporting element havingan inductive reactance, means for connecting the junction point of saidelements to a transmission line having a predetermined characteristicimpedance, the reactances of said elements being selected to match theimpedance of said antenna at said junction point to the impedance ofsaid line, and a plurality of quarter Wave conductors connected to theend of the outer conductor of said supporting element adjacent saidradiating element, and perpendicular to the axis of said supportingelement.

11. A high frequency antenna comprising a radiating element connected toa reactive concentric supporting element, means for connecting thejunction of said antenna and said supporting element to a transmissionline of predetermined characteristic impedance, the length of saidradiating element and the reactance of said supporting element being soselected that the'impedance of said antenna and supporting element atsaid point of connection is equal to said predetermined characteristicimpedance, a pluralityof quarter wave conductors connected to the end ofsaid supporting element adjacent said radiating element, and areflecting conductor mounted on the end of one of said quarter waveconductors and parallel to said radiating element.

12. A high frequency antenna comprising a radiating element and aconcentric conductor supporting element, said radiating element being alinear continuation of the inner conductor of said supporting element, atransmission line connected to the junction point of said members, and aplurality of quarter Wave conductors connected to the outer conductor ofsaid supporting member at the end adjacent said radiating member, andperpendicular to the axis of said members.

13. A device of the character described in claim 12 in which theelectrical lengths of said supporting and radiating members are lessthan a quarter wave at the operating frequency and of equal and oppositereactance.

14. A device of the character described in claim 2 which includes meansconnected to the end of the outer conductor of said supporting elementadjacent said radiating element for establishing an effective ground forsaid radiating element.

GEORGE H. BROWN.

